In explorations of the influence of serotonin on neuroendocrine function, prolactin, growth hormone and cortisol concentrations were all found to be elevated in rhesus monkey plasma following intravenous administration of a selective serotonin receptor agonist, m-chlorophenylpiperazine, across a six-fold dosage range. These hormone changes were either completely blocked (in the case of prolactin and growth hormone) or partially inhibited (cortisol) by low concentrations of the serotonin antagonist, metergoline.